1. Field of the Invention
This invention relates to a method and device for stabilizing a deteriorating spinal motion segment composed of two vertebrae adjacent a disc.
2. Description of the Related Art
Low back pain is a medical condition affecting millions of humans. It is the most common cause of disability for the middle aged working population. In addition to the discomfort and pain experienced by the individual, substantial costs are borne by society, including costs for the diagnosis and treatment, and the cost of payments for disability benefits. Lost productivity is more difficult to quantify, but we know that it is substantial. Any new knowledge that leads to a greater understanding of the causes, prevention, or treatment of the low back pain is a worthwhile and important contribution to the welfare of individuals and society in general. Any intervention that reduces or eliminates symptoms would constitute a significant breakthrough. That discovery would improve the quality of life for countless individuals and reduce the expenditure of scarce societal financial resources.
Recent medical evidence indicates that the symptoms of low back pain are most commonly caused by pathologic changes occurring in what is called the "spinal motion segment." The spinal motion segment consists of a unit of spinal anatomy bounded by two vertebral bodies, containing those bodies and the interposed intervertebral disc, as well as the attached ligaments, muscles, and the facet joints.
The disc consists of the cartilaginous end plates at the top and bottom of the vertebral bones, the annulus fibrosis running circumferentially around the nucleus, and the nucleus itself. In normal discs, the nucleus acts as a damper to cushion applied loads, thus protecting the other elements of the motion segment. The nucleus contains hydrophilic (water attracting) mucopolysaccharides and a small amount of fibrous strands. The nucleus is incompressible. It responds to compression forces (as in sitting, standing walking and lifting) by bulging outward against the vertebral end plates and the annulus fibrosis. Mechanical forces acting on the disc are substantial, i.e. approaching 1000 lbs/in.sup.2 when a heavy load is lifted.
The annulus consists of a woven/fabric of collagen fibers and a smaller amount of elastic fibers, both of which are extremely strong in resisting tension forces. Unfortunately the annulus is not very strong when it is required to withstand compression and shear forces. The vertebral end plate can deform slightly by bulging into the vertebral bodies, as long as the applied forces are not great, and a long as the force is dampened by the bulging of the annulus.
Disc Degeneration
The primary cause of low back pain is degeneration of the lumbar discs. For reasons that are not perfectly clear, many if not most humans develop pathologic changes in the nucleus as they approach middle age. In about one third of individuals, the process is painless. In the other two-thirds, the disease causes pain varying from mild and intermittent to severe and constant. As the nucleus begins to dry up, so to speak, it fragments, and looses its' ability to act as a cushion. This process is usually slow, but may be rapid if the disc is injured by the application of a very great load; as may occur during an accident or by the lifting of a great weight, or as a result of the combination of forces while stooping, bending, twisting or lifting.
This condition of disc dehydration is sometimes called "isolated disc resorption." Dehydration of the nucleus reduces its ability to act as a cushion. As a result, loads are transferred to the annulus of the disc and to the facet joints. These later structures are not capable of withstanding the applied compression and torsional loads, and they gradually deteriorate. Effects of this process include narrowing of the interspace, bony spur formation, fragmentation of the annulus, fracture and deterioration of the cartilaginous end plates, and deterioration of the cartilage of the facet joints. The annulus and facet joints loose their structural stability while subtle but pathologic motions occur between the spinal bones--a condition known as "segmental instability" occurs. The process is sometimes retorted to as the "degenerative cascade." Breakdown products of the disc and facet joint, including macroscopic chunks, microscopic particles, and noxious chemical substances build up. These breakdown products stimulate sensitive nerve endings in and around the disc, producing low back pain and sometimes, sciatica. Affected individuals experience muscle spasms, reduced flexibility of the low back, and pain when ordinary movements of the truck are attempted; e.g. stooping, bending, lifting, standing, walking, or even rolling over in bed. Simple and effective cures do not exist. The process is irreversible. Fortunately, in some if not most cases, as the disease continues to run its' course, the body eventually re-stabilizes the disc by stiffening the joints of the motion segment. Then the pain may decrease or cease altogether. Unfortunately, in a significant number of individuals (10-15%) the restabilization process does not take place. Even in the group where restabilization occurs, the process requires several years or even decades to complete. During that time the patients often experience recurrent bouts of disabling pain.
Direct observations by other investigators during the performance of lumbar procedures have confirmed the sensitivity of certain lumbar tissues. In addition, the most extensive direct observations of these phenomena to determine the exact source of lumbar pain have been performed and published by the inventor.
Clinically significant low back pain is derived, in the main, from pathologic stimulation of these, and only these structures: 1) the vertebral end plate, 2) the outer layers of the annulus fibrosis, and to a lesser extent, 3) the capsule of the facet joint. All of these structures produce typical low back pain when they are subjected to mechanical forces that exceed a certain threshold. In addition, it is likely, based on recent investigations into the biochemical milieu of the motion segment, that certain local chemical abnormalities may play a role in lowering the threshold for the stimulation of pain transmitting fibers.
Surgeons have devised several methods to stabilize the motion segment. These methods fall into the following general categories:
1) Decompression of the spinal canal by removal of part or all of the remaining nucleus, sometimes called "partial discectomy" or "discectomy." PA1 2) Decompression of the spinal canal by removal of non-nuclear tissues., e.g. ligamentum flavum removal or bone removal, sometimes called "laminotomy" or "laminectomy." PA1 3) Spinal fusion by a great variety of techniques.
Fusion consists of the preparation of the existing bone surfaces by decortication (scraping the surface of the bone) and the deposition of additional bone onto the prepared surface. The fusion may be posterior (from the back side of the patient) or anterior (from the front side of the patient). The bone may be cortical (hard bone) or cancellous (soft bone) or a combination of the two (cortico-cancellous). Great controversy exists regarding the preferred method of performing these fusions for various conditions of the spine. Sometimes non biological material are used to augment and support the bone grail (fixation systems). Sometimes the fixation is performed from the posterior route (posterior fixation), or from the anterior route (anterior fixation), or even both sides (anterior posterior fixations or circumferential fusion).
Bagby was the first to describe a method of stabilizing the spine by interbody fusion using a rigid, hollow housing containing bone graft, wherein the bone graft is contained entirely within the housing or implant. My modification of that method of spinal fusion (U.S. Pat. No. 5,015,255) attempts to accomplish a fusion with less damage to surrounding tissue and earlier stabilization of the motion segment, and using special tools for safe insertion. Even that method and many others similar to it require the placement of devices from both sides of the spine and the installation of a fairly large implant that will not fit through small portals. Examples include the implants described by Brantigan in U.S. Pat. No. 5,192,327; 4,878,915; 4,834,757 and 4,743,256. Two types of expandable, bone containing implants have been described. One is that of Kuslich, U.S. Pat. No. 5,059,193 consisting of a cylindrical device that expands to from a reinforced arch when it is expanded in the interbody space. Kuslich U.S. Pat. No. 5,059,193 uses a netting to prevent its ribs from sinking into soft vertebral bone. The other uses a memory metal (nickel-titanium alloy) that expands in the interspace when the temperature changes. Turn buckle type implants of Razian, U.S. Pat. No. 4,401,112 and Ogilvie, U.S. Pat. No. 4,636,217 expand, but do not contain graft within a housing.
Arthur Steffee describes an artificial disc with an elastomeric core between flat rigid plates in U.S. Pat. No. 5,071,437. An artificial disc using a spring is described in Kostuik et al, U.S. Pat. No. 4,759,769. Casey Lee et al, in U.S. Pat. Nos. 5,171,281 and 4,911,718 describe intervertebral disc spacers. Ray et al in U.S. Pat. No. 4,904,260 describes a disc capsule which must block passage of human cells. Bao et al in U.S. Pat. No. 5,192,326 describes a replacement for the nucleus filled with hydrogel beads. None of these encourage or allow through-growth of living cells.
The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is "prior art" with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 C.F.R. .sctn. 1.56(a) exists.